JPH01188471A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To obtain the subject sintered body having superior strength at high temp. by mixing Si3N4 powder with a specified amt. of mullite powder and Y2O3 powder as sintering assistants, molding the mixture and sintering the molded body in a nonoxidizing atmosphere. CONSTITUTION:94-99.2wt.% Si3N4 powder based on alpha-Si3N4 and having 0.1-1mum particle size is mixed with 0.8-6wt.%, in total, of high purity mullite powder and Y2O3 powder each having a particle size equal to or smaller than that of the Si3N4 powder in 2/1-1/1 ratio of mullite/Y2O3. The mullite powder consists of 71.6-72.0wt.% Al2O3 and 28.0-28.4wt.% SiO2, has >=99.9% purity and hardly contains impurity metals. The mixture is molded to a prescribed shape and the molded body is sintered at 1,650-1,850 deg.C in a nonoxidizing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a silicon nitride sintered body, and more particularly to a method for manufacturing a silicon nitride sintered body that has excellent strength at high temperatures.

[Prior Art] Silicon nitride sintered bodies have been used in various fields in recent years because they have excellent heat resistance, thermal shock resistance, and strength, and high corrosion resistance against nonferrous molten metals.

However, since it is difficult to sinter using silicon nitride powder alone, methods for producing a silicon nitride sintered body using various sintering aids have been proposed.

For example, in Japanese Patent Publication No. 49-21091, alumina (
Japanese Patent Publication No. Sho 52-3649 describes a manufacturing method using a group A to 3) and yttrium oxide (Yz03) as a sintering aid; Japanese Patent Publication No. 52-45724 discloses alumina, silicon oxide (Si02) and titanium oxide (Ti).
A manufacturing method using Oz> as a sintering aid is disclosed. In addition, as other compounds, magnesia (
MqO), zirconia (ZrO2), or MqAl
zOa and the like are known, and one or a combination of two or more of these compounds and the compounds disclosed in the above-mentioned publications are used as a sintering aid.

[Problems to be Solved by the Invention] The various oxides described above react with silicon oxide present as an oxide film layer on the surface of silicon nitride particles by heating to generate a liquid phase. It is believed that this promotes mass transport and improves the density of the sintered body. Therefore, the special public interest public corporation Sho 52-4
As seen in Japanese Patent No. 5724, addition of silicon oxide is also effective in promoting sintering. However, silicon oxide remains between crystal grains as silicate glass after sintering, and
This causes a decrease in the mechanical strength of the sintered body at higher temperatures. Therefore, 45kg/mm at 1200℃
It has been difficult to produce a sintered body with excellent strength at high temperatures, such as a bending strength of 2.

In addition, in order to crystallize the glass phase generated between the above-mentioned oxide and silicon oxide, the above-mentioned Japanese Patent Publication No. 52-457
No. 24, etc., discloses a method of performing heat treatment at a predetermined temperature after sintering. However, heating the sintered body again
There are problems in terms of man-hours and energy.

The present invention has been made in view of the above circumstances, and as a result of intensive research, it has been found that the type and amount of sintering aids can be used to eliminate the need for reheating and to produce a silicon nitride sintered body with high strength at high temperatures. The present invention was completed by finding an appropriate value for rF1.

[Means for Solving the Problems] The method for manufacturing a silicon nitride sintered body of the present invention includes silicon nitride powder 94
~99.2 weight M% and mullite (3AJ2203/2S
a molding step of mixing a total of 10.8 to 6% by weight of yttrium oxide (Y2O2) powder and yttrium oxide (Y2O2) powder to form a molded body into a predetermined shape; and heating and sintering the molded body in a non-oxidizing atmosphere. It is characterized by a sintering process of sintering.

As the silicon nitride powder, α-3i3N+ is usually used, but it may contain β phase. The particle diameter is preferably 0.1 to 1 μm as in the conventional case.

The greatest feature of the present invention is that mullite powder and yttrium oxide powder are used together as sintering aids. It is desirable to use mullite of high purity, which contains almost no impurity metals. Among them, A sentence go37
1.6-72.0 Weight and S i Ot 28. O~
28.4 fold ff1% T-1 purity of 99.9% or more is optimal. This mullite can be formed by a method of heating a sillimanite group mineral, a method of synthesizing an aluminum compound and a silica compound by heating them simultaneously, or the like. The particle size is preferably about the same as or smaller than that of 5i3Nn powder of 0.1 to 1 μm.

Yttrium oxide that has been conventionally used as a sintering aid can be used as is. The particle size is preferably about the same as or smaller than that of 5isN* powder of 0.1 to 1 μm.

The total amount of mullite powder and yttrium oxide powder is 0.8
It is mixed with silicon nitride powder so that the weight is ~6%.

If this total amount is less than 0.8% by weight, the sintering temperature will rise, causing sublimation and decomposition of silicon nitride. Also 6
When the amount exceeds % by weight, the strength of the obtained sintered body at high temperatures decreases.

The mixing ratio of mullite and yttrium oxide is preferably in the range of 221 to 1:1 in terms of weight ratio. Outside this range, the strength at high temperatures may decrease.

The molding step is a step in which the silicon nitride powder, mullite powder, and yttrium oxide powder are mixed and then molded to form a molded body of a predetermined shape. Conventionally used molding methods such as compression molding and slip casting molding can be used.

The sintering process is a process of heating and sintering the compact formed in the forming process in a non-oxidizing atmosphere.

Note that the heating temperature is preferably in the range of 1650 to 1850°C. If the temperature is lower than 1650℃, sintering becomes difficult;
If the temperature is higher than ℃, sublimation and decomposition of silicon nitride will occur.

[Operations and Effects of the Invention] In the method for producing a silicon nitride sintered body of the present invention, mullite and yttrium oxide are used in combination as sintering aids. This promotes sintering and improves strength at high temperatures. The reason for this is not clear, but during sintering, YtO3
-Aλ203-8iO2-based liquid phase is generated, promoting sintering even when added in a smaller amount than before, and when cooling solidifies, silicon oxide crystallizes as mellite or mullite and remains at grain boundaries. It is thought that the strength at high temperatures is improved.

That is, according to the manufacturing method of the present invention, a silicon nitride sintered body having high density and excellent strength at room temperature and high temperature,
It can be manufactured easily and reliably without increasing man-hours and energy.

[Example] Hereinafter, this will be explained in detail using examples.

(Example 1) (1) Molding process 97% α-3i3N4 powder with an average particle size of about 0°3 μm obtained by thermally decomposing diimide and an average particle size of about 0.4 μm.
1% by weight of Y2O3 powder of m and 2% by weight of high purity mullite powder with an average particle size of 0.15 μm are mixed together with ethyl alcohol in a resin ball mill for 72 hours. In addition,
The 5t3Na powder contains 1.6% by weight of oxygen and 20 oppm or less of metal impurities. Impurities are contained in the Y2O3 powder in an amount of 0.1% by weight or less. Mullite is also produced by aluminum improxide (Affi (OC)).
2H)3) and tetraethoxysilane (Si (OC2
It was synthesized by the sol-gel method using H5)4) as a raw material, and the results of X-ray diffraction revealed that the crystalline phase mullite (3A tO3 2Si
Oz).

Also, as a result of chemical analysis, AJ203 is 71.6-72.0
Weight %, 5iOz is 28.0 to 28.4 weight %,
Impurities are 0.03% by weight or less.

Ethyl alcohol is removed by distillation from the above mixture, and after drying by further heating to 150°C, primary molding is carried out at a pressure of 100 ka/cm', and then isostatic pressing is carried out at 3000 kg/cm' to form a molded article of a predetermined shape. did.

(2) Sintering process This molded body is heated to 1760°C in nitrogen gas at a temperature increase rate of 2°C/min and a pressure of 1 atm, and after reaching 1760°C, it is further pressurized to 9.5 atm. It was held for 4 hours and sintered.

(3) The sintered body obtained by the test has density, bending strength at room temperature, and 1
The bending strength at 000°C and 1200°C was measured and the results are shown in the table. Note that the density was measured by the Archimedes method using n-butyl alcohol, and the bending strength was determined by J
Measured according to 15-R1601. The bending strength at high temperature was measured under 1 atmosphere of nitrogen gas using a silicon carbide jig.

From the table, the sintered body obtained by the manufacturing method of this example is:
It can be seen that it has a density of 98.8% of the theoretical density and has excellent strength at high temperatures. Note that most of the pores in this sintered body were closed pores.

(Other Examples and Comparative Examples) The blending amounts of mullite powder and yttrium oxide powder were varied as shown in the table, molded and sintered in the same manner as Example 1 NI941, and tested in the same manner, and the results are shown in the table. . From the table, the sintered bodies of the examples have a density of at least 95% of the theoretical density. Also, if the mixing ratio of mullite and yttrium oxide is in the range of 2 = 1 to 1:1, the temperature will rise to 1200℃.
It also shows a high bending strength of 45 kG/mm2 or more, and is particularly excellent in strength at high temperatures. Furthermore, it can be seen that the sintering temperature becomes higher as the total amount of both decreases.

Patent applicant: Toyota Motor Corporation Agent: Patent Attorney Hiroshi Okawa

Claims (1)

[Claims] (1) A molded product of a predetermined shape by mixing 94 to 99.2% by weight of silicon nitride powder and a total amount of 0.8 to 6% by weight of mullite (3Al_2O_3.2SiO_2) powder and yttrium oxide (Y_2O_3) powder 1. A method for manufacturing a silicon nitride sintered body, comprising: a molding step of molding the molded body; and a sintering step of heating and sintering the molded body in a non-oxidizing atmosphere.